1. Field of the Invention
The present invention relates to a tendon of a tension leg platform (hereinafter referred to as the TLP) for producing offshore petroleum and gas. The present invention also relates to a method for electrical corrosion-protection for the tendon of the TLP.
Tendons are members used for anchoring the platform of a TLP to the sea bottom and are made of the coupled steel pipes.
2. Description of the Related Arts
Recently, investigations into petroleum and gas resources have extended into areas where the sea is deep. The problems to be solved at present are how to provide a structure which can be installed offshore at a water depth of 400.about.1000 m as a conventional scaffolding type-offshore structure is considered to be unable to meet the requirements for an offshore structure installed at such a water depth. Various offshore structures, said to exceed the limitations of the conventional scaffolding type, have been proposed. The TLP is considered to be one of the most promising of the structures which meet the requirements mentioned above and, hence, practical utilization thereof has begun. In this regard, steel pipes have not been used heretofore for anchoring a platform or the like to the sea bottom in deep areas. In addition, since the steel pipes of a tendon are extremely difficult to maintain, the steel material used for tendons must have a high mechanical durability. Another problem is how to protect the tendons from corrosion for a target life of from 20 to 30 years.
The Japanese technical journal "Piping and Plant" (March, 1981) illustrates one example of corrosion-protection of an offshore structure (c.f. page 34, right column, lines 6-9 and Table 4). That is, the parts of tendons immersed in seawater are galvanically corrosion-protected, and the upper parts of tendons above the ebb and flow level have a thin coating applied on a zinc-rich paint undercoat. When this corrosion-protection method is applied for tendons which are immersed in seawater, the tendons are galvanically corrosion-protected over their entire length and surface. Such an application will now be considered in more detail.
A tendon is manufactured by connecting steel pipes. In order to attain a uniform corrosion-protection potential over a length of 400 m or more, a galvanic current-anode (sacrificing anode) system, in which the power sources are dispersed, is more advisable than an external power-system using a centralized power source. Assume that the tendons are corrosion-protected for service over 30 years in a southern area of the northern Sea by using aluminum anodes, each of which is attached to a steel pipe 500 mm in diameter and 12 m in length, the net weight of each aluminum anode is presumably 340 kg.
Another approach, i.e., using only the paint coating system, is now described. Long term durability of a coating when immersed in sea water cannot be expected, when the coating must be thin enough to be applied on the upper part of a conventional offshore structure above sea-level, since such coating is likely to be damaged. A coating life 30 years can be realized if the coating is thick and thus will resist damage. The thick coating involves, however, a problem of whether or not the coating can be thickly applied on the steel pipes, in situ, during their installation off shore. The thick application appears to be unlikely, since the installation must be conducted continuously and this can be carried out only when the sea is calm. Another problem involved is that, although a coating can be formed in the factory on a major portion of a tendon, the screws used for coupling the steel pipes remain uncoated, and further, if the coating is preliminarily applied on the screws, it may be damaged by a fastening jig, e.g., a tongue for fastening the couplings to the steel pipes. Still another problem is that such a coating is generally consiered to be less durable than the galvanic corrosion-protection. A further problem is that the coating, which may be selectively applied on the straight section of a tendon or be extended to the coupling parts of a tendon, has a low durability at the edges thereof.
"Ocean AGe" (December, 1983) discloses that corrosion-protection of tentons exxclusively by the flame sprayed aluminum (FSA) process was tentatively conducted. However, a wait of several years would appear necessary to decide whether or not such protection is appropriate.
As is described above, an appropriate long-term corrosion protection is not attained by readily applying the prior art to the corrosion protection of the tendons. In summary, the corrosion-protection of tendons should fulfull the following requirements.
A. A corrosion-protection coating should have a durability and a damage resistance conforming to parts of the tendons.
B. Deterioration of a corrosion-protection coating should not occur at the edges of coating.
C. An anode of the galvanic corrosion-protection system should be as small in size as possible.
D. An anode should be connected to the tendon in a durable manner over a long period of time.
E. Steel pipes should be rust-proof treated prior to installation of tendons.
F. Application of a coating should be omitted, in situ, at installation of tendons.
The prior art of galvanic corrosion protection is now further described.
Unlike the conventional scaffolding structure, the tendons are subjected to great tension in sea water, since the TLP is in a state of continual movement due to wave action, and thus fatigue corrosion of the tendons is likely to occur. The galvanic corrosion-protecting method is effective for drastically lessening fatigue corrosion, as described in OTC paper No. 4449 (May, 1983) (c.f. FIG. 7). The galvanic corrosion-protection therefore can be advantageously used for protecting tendons, irrespective of whether it is used alone or in combination with the coating method. However, on the question of whether galvanic corrosion-protection is reliable, "Ocean Age" (December 1983) refers to the fact that fatigue corrosion is liable to occur at the parts of a galvanic-current anode where it is welded to the steel pipe (c.f. from page 54, right column, line 17 to page 55, left column, line 6 and FIG. 9). This fatigue corrosion occurs because, since a galvanic current-anode, which is a small-sized member, is locally welded, the galvanic current-anode is liable to harden and the stress is liable to be concentrated on the fixed ends of the galvanic current anode. According to a conventional galvanic current anode system, the core metal of the anode is fillet welded to the steel pipe to be protected. If this fillet welding is readily applied for the corrosion protection of tendons, not only would serious fatigue corrosion occur, but also the galvanic current anodes per se would fall off of the tendons during the service. Accordingly, when the galvanic current-anode system is applied for corrosion-protecting tendons, the galvanic current-anodes must be attached to the tendons in such a manner that the fatigue corrosion characteristic of the tendons are not seriously influenced and further, that the galvanic current-anodes do not fall off after a long period of time. Unless such attachment is accomplished, the intended corrosion-protection can not be attained, but instead, fatigue corrosion would be caused by the attachment of the galvanic current-anodes.